The other term is the equivalent of the Einstein–Hilbert action, as extended to all 5 dimensions.
The model was proposed in part in order to reproduce the cosmic acceleration of dark energy without any need for a small but non-zero vacuum energy density.
However, the theory remains interesting because of Dvali's claim that the unusual structure of the graviton propagator makes non-perturbative effects important in a seemingly linear regime, such as the solar system.
In 2008, Fang et al. argued that recent cosmological observations (including measurements of baryon acoustic oscillations by the Sloan Digital Sky Survey, and measurements of the cosmic microwave background and type 1a supernovae) is in direct conflict with the DGP cosmology[3] unless a cosmological constant or some other form of dark energy is added.
[4] However, this negates the appeal of the DGP cosmology, which accelerates without needing to add dark energy.